The characteristics of furnaces required in the case of incineration are quite different from those required for carbonization. For incineration, the materials should be comparatively violently agitated, in a generous supply of air, and subjected to comparatively high turbulence of air flow, during the entire period of combustion, in order to bring about the most complete reduction of the materials to ash as is reasonably possible. On the other hand, for carbonization, the materials may be tumbled for thorough exposure to the air, but should not be agitated in any extreme manner, since such agitation would result in the lowering of product yield. Also, the air supplied for carbonization of the material must be small enough to contain an insufficient quantity of oxygen for causing full combustion of the feed material, so that only partial thermal distillation of the material, producing various carbons, can occur. The flow paths of the material and the air must be carefully controlled and regulated so that only this sub-stoichiometric proportion of oxygen can reach the material being carbonized.
Also, in even the most efficient incinerating furnace, some unburned but combustible gases, as well as incombustible solid particulate matter, will exit from the principal combustion chamber of the furnace. Likewise, the same is true in carbonization furnaces, with the additional fact that carbonization furnaces will inherently produce large quantities of black, noxious smoke, which is highly objectionable as an atmospheric pollutant. In general, a carbonization furnace will produce a relatively large quantity of combustible gases, and a relatively small amount of particulate solids, in its effluent, as compared to an incineration furnace. As a result, it is common in either type of furnace to provide an afterburner of one type or another, having the object of both producing further combustion of any combustible components of the particulate matter, disposing of any remaining incombustible components of the particulate matter, as well as ash, in such a manner that it can be collected and disposed of within the system, and of burning any combustible effluent gases substantially completely, all to the end that the eventual effluent entering the atmosphere is "clean", and permissable under strict environmental protection regulations. However, the requirements for efficient afterburner operation for incineration furnaces also differ from those for carbonization furnaces. A carbonization afterburner, receiving principally only unburned gases, requires principally only an additional quantity of air and oxygen to support combustion of the gases, and some means for retaining the gases for a sufficient time period to complete the combustion thereof. An incineration afterburner, on the other hand, requires not only additional air and retention time, but also extreme turbulence in view of the solid particulate material entering it. Since such turbulence would carry some of the ash and other solid matter through the afterburner and discharge it to the atmosphere, the incineration afterburner should include a zone of quiet, non-turbulent air from which said solid matter may drop into a collection receptacle.